Carbon nanotubes (CNTs) are known for their extraordinary properties. For example, their strength is approximately 100 times that of steel, their thermal conductivity is approximately as great as that of diamond, their thermal stability reaches as high as 2800° C. in vacuo, and their electrical conductivity can be a multiple of the conductivity of copper. However, these structure-related characteristics are frequently only obtainable on a molecular level when carbon nanotubes can be distributed homogeneously and the greatest possible contact between the tubes and the medium can be produced, that is to say when the tubes can be rendered compatible with the medium and accordingly stably dispersible. With regard to electrical conductivity it is further necessary to form a network of tubes in which the tubes are ideally in contact or sufficiently close only at the ends. The carbon nanotubes are thereby to be present in as isolated a form as possible, that is to say without agglomerates, in a non-aligned manner and in a concentration at which such a network is able to form, which is reflected in the sudden increase in the electrical conductivity in dependence on the concentration of carbon nanotubes (percolation limit). An example of the direct use of conductive dispersions is conductive inks (see e.g. EP-A 1514280). Excellent dispersion and isolation of the carbon nanotubes is also required to achieve improved mechanical properties of composites such as, for example, in reactive resins such as epoxides, because larger agglomerates lead to fracture sites (Zhou, eXPRESS Polym. Lett. 2008, 2, 1, 40-48), and an impairment of the mechanical properties of such composites then tends to be observed.
For commercial applications, therefore, the incorporation of CNTs into liquid vehicles is of interest but is also a requirement. After their preparation, CNTs are present in the form of primary agglomerates. These primary agglomerates, which can have an order of magnitude of up to several millimeters, are initially not suitable for commercial use. In fact, they must be broken up so that the CNTs are present in isolated form and are able to form a stable dispersion and can be applied to the surface to be treated, for example in the form of thin layers. Isolated CNTs are also required to achieve desirable properties such as, for example, electrical conductivity.
For the successful preparation of stable dispersions of carbon nanotubes, therefore, the complete breaking up and unbundling of carbon nanotube agglomerates and—frequently—suppression of the high tendency of carbon nanotubes to re-aggregation is crucial if it is desired, by their use, to make a material electrically conductive, for example, and/or better in mechanical terms. Such dispersions must have different properties depending on the field of use. For example, for the use of inks in the inkjet printing process it is desirable for the residual agglomerate size to be sufficiently small that the nozzles do not become blocked. The same is true for a screen printing process, because agglomerates that are too large can lead to the formation of bridges on the screen and accordingly to blockages.
At the same time, however, a high concentration of CNTs in the dispersion is also desirable, in the case of conductive inks, for example, in order to make the printing operation as efficient as possible. There are two reasons for this: The wet layer thickness that can be achieved in a single step is limited with typical printing processes. The amount of CNTs which can be applied in a single printing step, and accordingly also the achievable conductivity per unit area, is therefore proportional to the CNT concentration in the dispersion. If a particular surface conductivity is required, it can require several printing operations, depending on the requirement and the printing process, which increases the outlay and may lead to problems with the accuracy of the printed structures. In CNT-containing starting products, a high concentration is desirable so that an adequate concentration can also be achieved in the end product.
It is additionally important for industrial use that a CNT dispersion is stable to the sedimentation of particles over a period of at least six months. For industrial applications, it is necessary to produce large amounts of dispersion, reaching the tonne scale. The production of such large amounts is not described in the literature.
CNT-containing dispersions can be prepared by various known techniques. Techniques known to the person skilled in the art are described, for example, in “Dispersion of Carbon Nanotubes in Liquids”, Journal of Dispersion Science and Technology, Volume 24, Issue 1 Jan. 2003, pages 1-41.
The techniques presented there are:                Dispersion with ultrasound: This process is very popular for laboratory processes but has the disadvantage that the required energy inputs are very high and the performance of the available ultrasound devices is technically limited, so that industrial production is scarcely possible. In addition, the energy input is concentrated very locally, and broad particle size distributions result. With higher degrees of filling with CNTs, the increase in viscosity has the result that the mechanism of ultrasonic dispersion, which is based substantially on cavitation, is greatly diminished.        Ball milling: As stated in the article, this process has the disadvantage that the CNTs are greatly damaged, which has an adverse effect especially on properties such as conductivity.        Trituration: This method destroys the structure and hence the properties of the CNTs even more than ball milling.        High-pressure mixing: Dispersion in a valve for diesel engines (ASTM D5275) led to considerable destruction of the CNT structures.        
WO-A 2009/100865 discloses a process for the preparation of conductive aqueous formulations containing carbon nanotubes and at least one polymeric dispersing aid, comprising at least the steps:                a) optional oxidative pre-treatment of the carbon nanotubes,        b) preparation of an aqueous pre-dispersion by dissolving the polymeric dispersing aid in an aqueous solvent, introduction and distribution of carbon nanotubes in the resulting solution,        c) introduction of a volume-related energy density, preferably in the form of shear energy, of at least 104 J/m3, preferably of at least 105 J/m3, particularly preferably from 107 to 109 J/m3, into the pre-dispersion until the agglomerate diameter of the carbon nanotube agglomerates is substantially ≦5 μm, preferably ≦3 μm, particularly preferably ≦2 μm.        
For step c), the preferred use of a high-pressure homogeniser is disclosed, the pre-dispersion preferably passing through the high-pressure homogeniser several times. A disadvantage of this process is that the maximum concentration to be obtained in the formulation corresponds to the maximum concentration which can be established in the pre-dispersion. Accordingly, in Example 3 of the application, by three separate passes through a high-pressure homogeniser, a dispersion of 0.5 gram of CNTs purified with H2O2 in 95 grams of polyvinylpyrrolidone solution is disclosed. The viscosity at a concentration of just below 0.53 wt. % is already 1.68 Pa*s at room temperature and a shear rate of 1/s. This low concentration of CNTs has the result that large amounts of water must be evaporated off in order to obtain a conductive coating. Because the viscosity of dispersions of CNTs increases greatly as the concentration of dispersed CNTs increases, a markedly higher viscosity is to be expected for higher CNT concentrations, which considerably limits the possibilities for commercial use. The conductivity achieved after drying was 3000 S/m.
US 2005/0224764 A1 describes CNT dispersions which, after application to a surface and drying, are electrically conductive and, owing to their shear thinning properties, are suitable, for example, for screen printing. The dispersions contain a carrier material (water or an organic solvent), a polymeric binder, typically a dispersing aid. The conductivities described in this application correspond, after conversion, approximately to those of WO 2009/100865. It is disclosed that the CNT dispersions can contain from 0.1 to 5% CNTs. However, it is also described in US 2005/0224764 A1 that the viscosity increases as the content of CNTs increases. This is also the reason why the dispersions are prepared first by pre-dispersion, by means of ultrasound, of a dilute solution containing not more than 0.5% CNTs, and only then by subsequent concentration and further dispersion by means of a milling process, in which the CNTs are shortened. In the examples given in the application, the CNT content is a maximum of 3.5 wt. %, in most cases 2.5 wt. % or less, and higher concentrations cannot be established in satisfactory quality by the mentioned process, even if it were desirable in some cases for the reasons mentioned above.